In 95% of the tier lists, the bonusses are there to compensate for the weaknesses or lack of options you have when using these "FAAC type" builds.

If you want a fluffy, laid back game, tier lists are awesome. If you want to have a more competitive game, tier lists generally lack some punch compared normal builds.

But it's a great way to have a game that doesn't always use supertuned lists. If both you and your oponent decide to use a tier army, you can have some awesome games totally in line with the fluff (if you have sufficient models for it, tier lists often requires 2 or more units of the same kind).